This application is related to subject matter disclosed in:                U.S. provisional App. No. 60/257,218 entitled “Waveguides and resonators for integrated optical devices and methods of fabrication and use thereof” filed Dec. 21, 2000 in the name of Oskar J. Painter;        U.S. provisional App. No. 60/301,519 entitled “Waveguide-fiber Mach-Zender interferometer and methods of fabrication and use thereof” filed Jun. 27, 2001 in the names of Oskar J. Painter, David W. Vernooy, and Kerry J. Vahala;        U.S. provisional App. No. 60/335,656 entitled “Polarization-engineered transverse optical coupling apparatus and methods” filed Oct. 30, 2001 in the names of Kerry J. Vahala, Peter C. Sercel, Oskar J. Painter, David W. Vernooy, and David S. Alavi;        U.S. non-provisional App. No.10/037,966 entitled “Multi-layer dispersion-engineered waveguides and resonators” filed Dec. 21, 2001 in the names of Oskar J. Painter, David W. Vernooy, and Kerry J. Vahala, said application being hereby incorporated by reference in its entirety as if fully set forth herein;        U.S. provisional App. No. 60/334,705 entitled “Integrated end-coupled transverse-optical-coupling apparatus and methods” filed Oct. 30, 2001 in the names of Henry A. Blauvelt, Kerry J. Vahala, Peter C. Sercel, Oskar J. Painter, and Guido Hunziker;        U.S. provisional App. No. 60/333,236 entitled “Alignment apparatus and methods for transverse optical coupling” filed Nov. 23, 2001 in the names of Charles I. Grosjean, Guido Hunziker, Paul M. Bridger, and Oskar J. Painter;        U.S. provisional App. No. 60/360,261 entitled “Alignment-insensitive optical junction apparatus and methods employing adiabatic optical power transfer” filed Feb. 27, 2002 in the names of Henry A. Blauvelt, Kerry J. Vahala, David W. Vernooy, and Joel S. Paslaski; and        U.S. non-provisional App. No. 10/187,030 entitled “Optical junction apparatus and methods employing optical power transverse-transfer” filed Jun. 28, 2002 in the names of Henry A. Blauvelt, Kerry J. Vahala, David W. Vernooy, and Joel S. Paslaski.        This application is also related to subject matter disclosed in the following publications, each of said publications being hereby incorporated by reference in its entirety as if fully set forth herein:        P1) R. D. Pechstedt, P. St. J. Russell, T. A. Birks, and F. D. Lloyd-Lucas, “Selective coupling of fiber modes with use of surface-guided Bloch modes supported by dielectric multilayer stacks”, J. Opt. Soc. Am. A Vol.12(12) 2655 (1995);        P2) R. D. Pechstedt, P. St. J. Russell, “Narrow-band in-line fiber filter using surface-guided Bloch modes supported by dielectric multilayer stacks”, J. Lightwave Tech. Vol.14(6) 1541 (1996);        P3) Carl Arft, Diego R. Yankelovich, Andre Knoesen, Erji Mao, and James S. Harris Jr., “In-line fiber evanescent field electrooptic modulators”, Journal of Nonlinear Optical Physics and Materials Vol. 9(1) 79 (2000); and        P4) Pochi Yeh, Amnon Yariv, and Chi-Shain Hong, “Electromagnetic propagation in periodic stratified media. I. General theory”, J. Optical Soc. Am., Vol. 67(4) 423 (1977).        
Transverse optical coupling (also referred to as transverse coupling, evanescent optical coupling, evanescent coupling, directional optical coupling, directional coupling, transverse optical power transfer, transverse transfer) is discussed at length in several of the prior patent applications cited hereinabove, and the discussion need not be repeated herein. The efficiency of transverse optical coupling is determined by the degree of transverse overlap between an optical mode of the device and an optical mode of the waveguide (characterized by a coupling coefficient κ), by the propagation distance over which the modes overlap (i.e., interaction length L), and by the degree of modal index mismatch for so-called mode-interference transverse coupling (characterized by Δβ=β1−β2, the β's being the propagation constants for the respective optical modes). Techniques and devices for efficient transverse optical coupling between a fiber-optic taper and a modal-index-matched optical waveguide fabricated on a substrate have been developed recently and may find applicability in the telecommunications industry. Examples include semiconductor-based multi-layer-reflector (MLR) waveguides and/or resonators, as disclosed in earlier-cited application Ser. No. 10/037,966, and methods and apparatus disclosed herein may be suitable for other transversely-optically-coupled optical components as well. External-coupling waveguides for modal-index-matched transverse optical coupling between optical devices and transmission waveguides are disclosed in earlier-cited application Ser. Nos. 60/334,705; 60/360,261; and 10/187,030. Transverse-coupled components may include waveguides wherein confinement of waveguide optical modes is achieved by lower-index cladding layers surrounding a core, distributed Bragg reflection, reflection from metal coatings, reflection from dielectric coatings, reflection from multi-layer coatings, and/or internal reflection at an air/waveguide interface. In order to attain the full potential of many devices employing transverse optical coupling, polarization-dependent behavior of the transverse optical coupling and/or polarization-dependent optical propagation in one or more of the coupled optical components should preferably be suitably engineered for achieving the desired device performance.
In the field of optical-fiber-based telecommunications, optical signals are generated and launched into an optical fiber for transmission to another location. The optical signal may propagate over long distances (hundreds or even thousands of kilometers), usually carried by single-mode optical fiber. Such optical fiber does not typically preserve the polarization state of the propagating optical signal, which generally appears at the far end of the fiber in an arbitrary polarization state. As disclosed in the above-cited applications, the optical signal may be most efficiently transferred between the optical fiber and signal receiving and/or processing devices (which may be optical and/or electronic devices) by transverse optical coupling between a tapered section of the optical fiber and a modal-index-matched-optical-waveguide input section of the signal receiving/processing device. Other types of waveguides may also be employed for transverse-coupling.
Achieving modal-index-matched propagation of an optical signal within the fiber-optic taper and the waveguide may be achieved in a variety of ways. One particularly applicable technique involves the use of multi-layer reflector (MLR) optical components (waveguides and/or resonators) fabricated on a substrate, as disclosed in earlier-cited application Ser. No. 10/037,966. Such waveguides may be engineered so as to exhibit modal indices for supported optical modes that are near the modal index of the optical mode propagating in the fiber-optic taper. Precise modal-index-matching of the MLR optical component and the fiber-optic taper (and therefore also optimum transverse optical coupling) may be achieved by suitably precise design and fabrication of the MLR optical component (passive modal-index-matching) and/or by application of a control bias voltage across the MLR optical component, thereby using electro-optic properties of the MLR optical component to shift its modal index to match that of the fiber-optic taper (active modal-index-matching).
However, the two substantially orthogonally linearly-polarized optical modes supported by a MLR optical component (i.e., designated as TE and TM modes) typically have differing modal indices, making it unlikely that both modes could be modal-index-matched to the fiber-optic taper simultaneously. Since only one of these modes would be modal-index-matched, the optical signal power would be efficiently transferred to the MLR optical component from only one of two substantially orthogonally linearly-polarized optical modes of the fiber-optic taper. The rest of the optical signal (carried by the other linearly polarized mode) would remain mostly in the fiber-optic taper.
It is therefore desirable to provide apparatus and methods for enabling substantially complete transfer of an arbitrarily polarized optical signal between transversely-optically-coupled optical components. It is desirable to provide apparatus and methods for enabling substantially complete transfer of an arbitrarily polarized optical signal between a waveguide or resonator (including MLR waveguides and resonators) and a fiber-optic taper transversely-optically-coupled thereto. It is desirable to provide apparatus and methods for enabling substantially complete transfer of an arbitrarily polarized optical signal out of a fiber-optic taper and into one or more waveguides and/or resonators (including MLR waveguides and resonators) transversely-optically-coupled thereto.